1. Field of the Invention
The present invention relates to an image display device. More specifically, the present invention relates to an image display device including a polarizer and an optical film.
2. Description of the Related Art
Recently, the application of an image display device such as a liquid crystal display (LCD) or an organic electro luminescence (EL) display device has expanded, and the image display device has been used in a television, a monitor, a smart phone, a car navigation, a digital camera, digital signage, and the like.
In general, LCD includes a polarizing plate on both of a visible side and a light source side of a liquid crystal panel. In addition, an organic EL display device including a polarizing plate on a visible side of an organic EL panel has been made into a product in order to suppress reflection of external light.
In such an image display device including the polarizing plate, the exiting light becomes linearly polarized light, and thus, in a case where the image display device is observed by mounting polarized sunglasses, display darkens, and visibility may considerably deteriorate.
Therefore, in JP4888853B and JP2012-256014A, it has been proposed that excellent visibility is able to be obtained by using a polyester film having great optical anisotropy in a display device or the like even in a case where polarized sunglasses are used.
For example, in JP4888853B, a polyester film having retardation (Re) in an in-plane direction of 3,000 nm to 30,000 nm is disclosed. In addition, in JP2012-256014A, polyester film having retardation (Re) in an in-plane direction of 3,000 nm to 30,000 nm is disclosed in which Re/Rth≥0.2. Furthermore, retardation (Rth) in a thickness direction of these polyester films has a positive value.
On the other hand, it has been known that in a case where a film which has retardation in an in-plane direction of greater than or equal to a hundred nm and great optical anisotropy, such as a biaxially stretched polyester film, is used as a polarizer protective film, rainbow-like color unevenness (rainbow unevenness) occurs at the time of being observed by mounting polarized sunglasses, and visibility deteriorates. Light which exits from the image display device and is transmitted through a polarizer becomes linearly polarized light, but in a case where the polarizer protective film has large Re, the light exiting from the image display device has various phase variations at each wavelength by retardation of the polarizer protective film. In a case where the light exiting from the image display device is observed by mounting polarized sunglasses, different transmittance is obtained at each wavelength, and thus, the light is observed as being colored, and this causes rainbow unevenness. Further, even in a case where the polarized sunglasses are not mounted, the rainbow unevenness may be visible due to polarization dependency of reflectivity of the outermost surface of the polarizer protective film.
However, in the image display device disclosed in JP4888853B and JP2012-256014A, a light source has a continuous light emitting spectrum as with a white LED, and Re of the polarizer protective film is a sufficiently large value, and thus, even in a case where the transmittance is different at each wavelength at the time of observing the light exiting from the image display device by mounting polarized sunglasses, light having various wavelengths of a red color, a green color, and a blue color configuring a white color is transmitted, and thus, the light is entirely observed as white. That is, the rainbow unevenness is not visible. Furthermore, in a case where the image display device which includes an optical film having large Re is observed not only from a front surface but also from an oblique direction, retardation of the optical film on appearance may decrease compared to a case where the image display device is observed from the front surface, and at this time, the size of Re may become insufficient and the rainbow unevenness may be visible, but in JP4888853B and JP2012-256014A, even in a case where the image display device is observed from the oblique direction, an effective range of Re and Rth of the optical film is defined such that the optical film has large Re on appearance.